There exists two types of video disks. The CAV (Constant Angular Velocity) disk records signals at a constant angular velocity along the disk tracks. One video frame is recorded for each disk revolution. The CLV disk records signals along the disk tracks at a constant linear velocity of the disk. Thus, the number of video frames per disk revolution varies, increasing from the inner portion of the disk toward its outer circumference.
For a CAV disk there is recorded only one picture over each circumferential track from the inner circumference to the outer circumference of the disk. It is sometimes desirable to reproduce the recorded video at speeds other than the standard playback speed. For example, it may be desired to playback the video as a still picture by repeatedly retracing the same circumferential track. It is also desirable to be able to playback the video at a multiple of the standard playback speed, such as twice, three times or more generally n-times the standard playback speed. Slow motion playback is included in such multispeed operations.
To accomplish, for example, double speed reproduction, track jumping operations are performed wherein the disk player pickup jumps tracks formed spirally or concentrically on the disk in a manner to skip readout of every other track. The track jump operation takes place during a vertical blanking period occurring after a frame has been reproduced. A triple speed reproduction is performed by a jump operation in which the pick up jumps to the next track after the disk has rotated only one-half a revolution to reproduce one video field. Slow motion reproduction can be accomplished with playback at 1/2 standard playback speed. This is effected by reading out each track twice in succession.
Since in the CLV disk, a video signal is recorded at a constant linear velocity, reproduction of the disk is effected by varying the number of rotations per unit time of the disk player continuously from 1800 rpm to approximately 600 rpm in accordance with the movement of the pickup point of the pickup head from the inner circumference to the outer circumference of the disk. In a CLV disk, one picture (video frame) is stored over one track at the innermost circumference of the disk, with the number of frames per track increasing toward the outer circumference to the disk, with three frames stored on one circumferential track at the outermost circumference of the CLV disk. Accordingly, the recorded video frames are not synchronized with each other in the radial direction of the disk. That is, the vertical blanking periods following each field or frame do not coincide along a radius of the disk as they do in a CAV disk. Therefore, it is difficult to perform multispeed reproduction since the pickup point of the pickup head in a video frame before and after a track jump do not coincide and therefore synchronization is lost when multispeed operation is attempted with a CLV disk.
That is, since a CLV disk does not have the same number of frames recorded over that circumferential track, the multispeed reproducing operation for the CLV disk cannot be carried out by the same manner as that for the CAV disk. Assuming that the multispeed reproducing operation for the CLV disk is carried out by the same manner as that for the CAV disk, the playing speed at the outermost track is 2.64 times as much as a preset speed at the innermost track, because three frames are stored on the outermost track as mentioned above. This will be described in more detail hereinafter.
This loss of synchronization causes phase control of a timing servo system of the reproducing apparatus to be disturbed as a track jump operation is performed. As a result, there is a large variation in the time axis of a reproduced video signal. Further, the timing of the horizontal synchronization signals H and vertical synchronization signals V of the reproduced video signal become improper so that the synchronization of a TV monitor is not properly performed causing the reproduced picture to exhibit vibrations both horizontally and vertically.
Partial solutions to the aforementioned problems have been proposed in U.S. Pat. Nos. 4,609,948 and 4,763,205 owned by the same assignee and Published Unexamined Japanese Patent Application No. 205857/1988 by the applicant of this application. These publications describe a technique which makes it possible to rapidly correct the phase control of the time axis servo system which has been disturbed by the track jump operation. The technique conforms, in the time axis servo system, the phase of a reference signal with respect to a synchronization signal included in the signal read from the disk at the end of the jump operation to the phase thereof at the start of the jump operation.
Further, by using a frame synchronizer disclosed in U.S. Pat. No. 4,743,979 owned by the same assignee a video signal is converted to a digital signal with a clock synchronized with the reproduced video signal. The digital signal is stored at a predetermined position of a video memory and then read out using an external, stable clock. The horizontal synchronization signals H and the vertical synchronization signals V are synchronized with the external, stable synchronization signal, thereby solving the above-mentioned synchronization problem.
With the problems resulting from the track jump operations significantly reduced as described above, the playing operation and the track jump may be alternately performed, enabling multispeed reproductions.
However, if the multispeed reproduction is to be performed using a CLV disk in the same manner as it is performed when a CAV disk is used, the image reproducing speed varies with the radial position change of the pickup point of the pickup head when, for example, there is a single track jump from an information reading point in one track to the reading position on the next track after the jumped track as occurs when playback is to occur at twice the standard playback speed.